a. Field of the Invention.
The invention relates to transistor amplifiers and, more particularly, to a high fidelity, transistor audio power amplifier.
B. Prior Art.
Lowering distortion in transistor power amplifier circuits without comprising their transient response remains a primary problem in high fidelity audio systems. Until fairly recently, a typical technique for removing distortion components in linear amplifiers was to cascade many gain stages to form a circuit having enormous amounts of gain and then using negative feedback to control the system and correct for the many errors introduced by the large number of components.
While the sum of the distortions of these components may cause large complex nonlinearities, the correspondingly large amounts of feedback applied are generally more than equal to the task of cleaning up the performance with only one trade off; the high frequency performance of the system. Because each amplifying device also contributes its own high frequency rolloff, and because the sum of many of these rolloffs creates a complex, multi-pole phase lag, a system using large amounts of negative feedback tends to be unstable at high frequencies, resulting in phenomena popularly referred to as transient intermodulation distortion (TIM). There are two main prior art approaches to TIM reduction. The first prior art approach is to not require any high frequency performance of the circuit; that is, not to feed it high frequency signals that it cannot handle. While this solution works very well for many operational amplifier applications requiring only low frequency performance, it is judged to be unacceptable in high fidelity applications where high voltage frequency response is required beyond 100 kilohertz.
The second prior art approach involves simple circuits having few amplifying devices and low open loop gain. The simplicity and low gain allows the circuitry to respond to signals very quickly, thus eliminating transient problems, but does so at the expense of higher simple harmonic and intermodulation distortions. While not as offensive as TIM, these distortions present another problem and another approach is needed.
Previously, cascode circuit operation has been employed in high fidelity pre-amplifier, signal processing circuits and elsewhere. The reverse open-circuit amplification parameter is very much smaller for transistors connected in cascode than for a single common emitter stage, thereby yielding good stability for the circuit. A discussion of transistor cascode operation, including a model of the circuit may be found in the book "Electronic Devices and Circuits" by Millman and Halkias, McGraw-Hill, 1967, p. 355-357. Although cascode operation has been tried in portions of amplifiers, no known amplifier has utilized cascode operation through to the output stage.